1. Field of the Invention
This invention relates to printed circuit board (PCB) design. In particular, the invention relates to capacitance fabricated in the printed circuit board.
2. Description of Related Art
Mode compensation is one known signal quality technique in microwave PCB design. It is used to compensate for the different odd and even mode wave velocities through microstrip, where fields through air and dielectric cause the odd mode to be faster. The microwave application is for essentially analog signals and allows, for example, near-ideal directional couplers to be fabricated. In the present work, the mode compensation is used to help minimize the upward glitch seen on system memory data lines (i.e., when one or more lines are held low and the other data lines switch from low to high).
Mode compensation requires placing capacitance between adjacent signal traces, usually at the driver or receiver end of the trace. One way to implement mode compensation using the existing technology is to place discrete capacitors on the PCB, with highest priority to the receiver end of the signal traces. This method, however, is difficult and expensive. Placing discrete capacitors at the receiver end requires adding many discrete components, which add to the bill-of-materials (BOM) cost of the PCB. Furthermore, laying out a PCB to add sites to place these discrete capacitors is difficult because at the receiver, signal traces are often spaced 5 milli-inches (mils) apart while the most common capacitor size used in this application is 30 mils wide. Finally, placing discrete capacitors to implement mode compensation violates design-for-manufacturing (DFM) rules pertaining to component-to-component spacing, which leads to higher assembly fallout and more expensive finished PCBs.